DEEP SANDS
Midway along the Highway from Melbourne to Adelaide, between Nhill and Kaniva, the road crosses a series of parallel sandhills. Lawloit Ridge is the steepest of these phenomena, which are actually "stranded coastal dunes". They are remnants of a geological past when much of the Murray-Darling Basin lay under the Southern Ocean.
It was a time when global sea levels fluctuated according to a climatic cycle. Over millions of years, they fell when polar ice sheets froze (glacials) and rose when polar ice sheets melted (interglacials). The last period when marine flooding of southeastern Australia reached its maximum was approximately 6 million years ago (MYA).
2. 2016, CV Murray-Wallace, Coastal Landscapes of South Australia, Adel Univ Press, p 149.
3. 2022, Wimmera Catchment Management Authority, <wcma.vic.gov.au>
Since that peak, the ocean has periodically retreated, leaving behind with each fall an old coastline of raised sandhills. The most obvious of these are also the most recent. The South East of South Australia, when crossed east to west from Naracoorte to Kingston, presents a convincing record of marine regression.
There are 30 relict coastal dunes forming 13 distinct ranges across the plains. The youngest of these now lies submerged 50-100 metres below sea level, drowned in the warming that followed the Last Glacial about 20,000 years ago. The most recent exposed site is the Robe Range, deposited about 100,000 years before present (YBP). The oldest is West Naracoorte Range, dated 710,000 YBP (+/- 62,000 years).
Less obvious "stranded shorelines" are evident in the West Wimmera region of Victoria. At higher altitude than the SE of SA series, they are older, and have been degraded by weather and subsequent soil overlays. For example, Lowan Sand now makes up the surface of the Big and Little Deserts.
However, the remaining areas of the earlier formations can still be seen as they radiate out from the north and south sides of the Little Desert. The ages of these fainter arcs are difficult to establish. Dates from the Late Pliocene (2.6 MYA) to the Mid Pleistocene (780,000 YBP) have been suggested.
An advantage of understanding this 'earth-history' is that it describes a number of unique, separated environments. It may not explain every isolated patch of stringybark scrub, but it identifies their singular nature. To amplify the analogy, beach sand makes fundamentally impoverished soil.
Even when experiencing above average rainfall, these ancient shorelines lack the fine-grained structure to retain moisture. They are very "well drained". This leaching capacity contributes to the absence of important trace elements like copper, cobalt, zinc, potassium and phosphorus. Similarly, low organic matter means necessary concentrations of nitrogen and carbon are rarely available.
Stringybark scrubland is accurately defined as: "pockets of edaphic dry sclerophyllous woodland associated with deep nutrient-poor acidic sands naturally fragmented by open grasslands on richer clay soils". In short, the plant-life existing on these light ridges must be very tough, able to draw up and hold onto scarce nutrients where they can.
The Stringybarks
Brown Stringybark, Eucalyptus baxteri, and Desert Stringybark, Eucalyptus arenacea (Latin for "of sand"), are the dominant canopy trees of the deep sands in southwest Victoria and southeast South Australia. Commonly found on infertile rises, they were usually left uncleared by farmers who considered the type of country where they grew was useless for cropping or grazing.
Brown Stringybark tends to occur in the south of the region. It can be an upright tree with a single trunk (reaching up to 20 metres), or have two or three main branches spreading out from ground level. In exposed positions near the sea it is more shrub-like and is limited to 2 or 3 metres tall. Desert Stringybark is found further inland, developing as a multi-stemmed tree from 3 to 9 metres high (sometimes called a "mallee habit").
Both species have thick fibrous grey-brown bark that typically covers all the trunk and branches towards the leaves. Both develop a substantial lignotuber, which is a basal swelling of the plant stem at or beneath the ground surface (again, like "mallee stumps"). Lignotubers act as "resource sinks", storing carbohydrates and other nutrients for times of need. They also maintain a supply of bud-forming nodes that allow the stringybark to quickly reshoot after drought or fire.
2. Derived from research and doctoral dissertation by P Koch, 2005, 'Factors influencing food availability for the
endangered south-eastern Red-tailed Black Cockatoo in remnant stringybark woodland and implications for
management'.
Stringybarks occupy micro-environments susceptible to drying out and being burnt. Thick bark and lignotubers are useful adaptations for species survival in marginal habitats subject to fire. In 2007 a study was made of the burn-history of stringybark scrub scattered along the South Australian border in southwest Victoria. The sampling results from 161 examined sites revealed a close relationship between E. baxteri / E. arenacea and the frequency of bushfires.
Fire perimeters had been reliably recorded in the district for 45 years before the individual sites were tested. These maps showed an average fire-cycle of 34 years from natural blazes and prescribed burning. 76 of the 161 sites had been burnt more than once, and 36 of these had intervals between fires of less than 11 years.
1. Image of part of survey area reproduced from TF Duff et al, 2013, 'Managing multiple species or communities? Considering variation in plant species abundances in response to fire interval, frequency, and time since fire, in a healthy Eucalyptus woodland', Forest Ecology and Management, 289, p 396
Response curves were plotted for the stringybarks and associated species. It is important to note that the overwhelming majority of 'companion plants' were "resprouters" as well. Like the stringybark, they were able to survive fire by "vegetative recovery" (not "obligate seeders", killed by fire and reproducing only by seed).
The abundance and diversity of the scrublands was found to be unaffected by fire in the medium to long term. An indication of this was the measure of bare ground (inversely reflecting the rate of recovery by plants after fire). Areas of bare ground decreased as plants reoccupied the sites up until a turning point at 15 years. At this time the surface area of bare ground stopped declining and slowly began to increase (inferring that the area 'recovered' by herbage then began to decrease).
This impression was confirmed by the experience of Brown Stringybark, which reestablished relatively quickly after fire and extended its occupation during the next 15 years. Once time since the last fire reached this point, species-abundance plateaued, then began to fall.
In general terms, some of the plateauing and declining effect was due to normal organic growth. Because "cover abundance methods cannot discriminate between many smaller plants and fewer larger plants, it is likely that the high abundances soon after fire are due to the mass germination of seed. With increasing time since fire, attrition would reduce the number of individuals, resulting in a decline of cover".
There was, in fact, some variance in the way frequency of fire effected associated plants. Yaccas, Xanthorrhea australis, (grass-trees, 'black-boys'), are protected by a dense mat of dead needle-like leaves and their central stem is partly buried in the soil by this litter. A standard response to fire is to send up flowers on a long thin staple and so their abundance is increased. But because they are slow growers, this response to fire is compromised by the frequency of reburning.
Bracken, Pteridium esculentum (Austral bracken, 'bracken-fern'), on the other hand, did not demonstrate any response to the fire regime, neither to time since the last fire or short fire intervals. A quick coloniser of disturbed ground after disaster, its reproductive capacity is beneath the surface in its extensive and aggressive root system.
In terms of the dominant stringybark species though, E. baxteri was able to survive, even thrive, in circumstances of much shorter fire intervals than the historical average of 35 years. Beyond the frequency limit of 15 years, its renewal and restorative vigour seems to have been stimulated by regular burning.
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